Electronic packages and a method to improve thermal performance of electronic packages

ABSTRACT

An electronic package incorporating a heat-generating element which is thermally coupled to a heat-sinking member, through the utilization of a predetermined thermally conductive material, and wherein all of these components are placed in compression during package operation so as to resultingly improve the thermal performance of the electronic package. A method is set forth of improving the thermal performance of an electronic package through the intermediary of compressive forces being generated between the package components during package operation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of Ser. No. 08/923,008;filed on Sep. 3, 1997.

BACKGROUND OF THE DISCLOSURE

1. Field of the Invention

The present invention relates to an electronic package incorporating aheat-generating element which is thermally coupled to a heat-sinkingmember, through the utilization of a predetermined thermally conductivematerial, and wherein all of these components are placed in compressionduring package operation so as to resultingly improve the thermalperformance of the electronic package. Moreover, the invention isdirected to a method of improving the thermal performance of anelectronic package through the intermediary of compressive forces beinggenerated between the package components during package operation.

In essence, the thermal performance of an electronic package, which maybe constituted of a heat-generating element such as a semiconductorchip, and which is coupled to a heat-sinking member, such as a finnedheat sink, or the like, may be implemented through the utilization of aspecified thermally conductive material, and in which the thermalperformance is governed by the highest thermal resistance present in thepackage. Quite frequently, this thermal resistance is encountered at thesmallest area through which the heat which is generated must flow, andthis typically occurs in a region or a gap between the chip dissipatingthe power and the heat sink, a cover plate or heat spreader. Theresistance to heat flow in that direction is normally R=l/kA, where Rrepresents the resistance in C/W, k is the thermal conductivity of thematerial present in the gap and l is the thickness of that material.

Pursuant to the inventive concept, compression between the variouselectronic package components can be provided through the use of amechanical structure, such as springs or similar biasing elementsimparting compression to the electronic package components, or throughthe employment of two adhesives possessing different coefficients ofthermal expansion so as to, during operation of the electronic package,generate compressive forces improving the thermal performance of theelectronic package.

2. Field of the Prior Art

The concept of employing compressive forces, and various methods ofimparting such compressive forces to the components of electronicpackages, is well-known in this technology.

Thus, U.S. Pat. No. 5,548,482, to Hatauchi et al., describes a modulewhich is mechanically supported through the attachment thereof to aprinted circuit board. The patent states that the supports for a cardmounting the printed circuit board are attached by soldering, and thus,are able to support only limited stress above which the solder will tendto creep, especially at high operating temperatures. There is no cleardisclosure of any method or structure to provide for the enhancement ofthe thermal performance of electronic packages analogous to that of thepresent invention.

U.S. Pat. No. 5,602,719, to Kinion, describes a structure to provide forelectronic package level cooling, and not for chip levels, as disclosedby the present inventive concept. There is no disclosure of providingfor an improved thermal performance through the intermediary of anadhesive and applied pressure during electronic package operation. Thereis only a disclosure of a thermal grease and no suggestion of utilizingan adhesive, such as preferably a thermal adhesive, as contemplatedherein. Thermal grease tends to come out during operation because ofpackage movement.

U.S. Pat. No. 5,022,462, to Flint et al., describes a flexible finnedheat exchanger and is adapted to provide structure to replace TCM-like,multi-chip modules. Thermal enhancement is provided through the use of agrease, and not through an adhesive material, as provided by the presentinvention.

U.S. Pat. No. 5,581,442, to Morosas, describes a spring clip forclamping a heat sink module to an electronic module, and does notdisclose the particular use of an adhesive, such as a thermal adhesiveanalogous to the present invention.

Similarly, U.S. Pat. No. 5,595,240, to Daikoku et al., also describes acooling apparatus for electronic devices including a resilient springstructure located internally of a module, and there is no discussion ofthe effect of pressure acting on components of the electronic package toincrease the thermal performance of the package through the use of anadhesive, such as a thermal adhesive.

SUMMARY OF THE INVENTION

The present invention, as set forth hereinabove, thus, describes a novelelectronic package and a method of increasing or enhancing the thermalperformance of the electronic package through the utilization, pursuantto one embodiment, of a mechanical spring structure employed inconnection with an adhesive, preferably a thermal adhesive; and pursuantto a modified embodiment, the use of two thermal types of adhesiveshaving different coefficients of thermal expansion so as to ensure, inboth embodiments, the application of a compressive force or clampingpressure on the semiconductor chip.

Accordingly, it is an object of the present invention to provide a novelelectronic package constituted of a heat-generating element which isthermally coupled to a heat-sinking member, and employing predeterminedthermally conductive materials, wherein the heat-generating element,heat-sinking member and the conductive material are placed incompression during package operation.

A more specific object resides in the provision of an electronic packageof the type described herein wherein the compression during operation ofthe electronic package is imparted through the use of a mechanicalstructure in conjunction with a preferably thermal adhesive.

Another object of the present invention resides in the provision of anelectronic package of the type described utilizing two adhesives,preferably thermal adhesives, possessing different coefficients ofthermal expansion, so as to impart the required compression to theheat-generating element and to resultingly improve the thermalperformance of the electronic package during operation thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference may now be made to the following detailed description of thepreferred embodiments of the invention, taken in conjunction with theaccompanying drawings; in which:

FIG. 1 illustrates a generally diagrammatic representation of a firstembodiment of the electronic package pursuant to the invention; and

FIG. 2. illustrates, in a representation similar to FIG. 1, a modifiedembodiment of the electronic package utilizing two adhesives possessingdifferent coefficients of thermal expansion.

DETAILED DESCRIPTION

In essence, referring to FIG. 1 of the drawings, there is illustrated anelectronic package 10 in which a chip 12, such as a semiconductor chip,is positioned between a first plate 14 comprising a chip carrier havingsolder balls 16 on an opposite surface distant from the chip, and asecond plate 18 constituting a cover plate or a heat-sink. The chip 12is bonded to the chip carrier 14 at surface 20 thereof, and interposedbetween the opposite surface 22 of the chip 12 and the facing surface 24of the cover plate 18 is an adhesive, preferably a thermal adhesive 26.In order to provide an improved degree of compression during operationof the electronic package 10, as the heat rises in the semiconductorchip 12, the surface 24 of the cover plate 18 facing the semiconductorchip 12 or adhesive 26 may be slightly concave in configuration.Mechanical springs or clips 30 are provided at opposite sides of thesemiconductor chip, and may be equipped with internal stiffener elements32, so as to impart a constant clamping effect to the semiconductor chip12. During operation of the electronic package 10, the adhesive 26 whichis interposed between the cover plate 18 and the semiconductor chip 12will exhibit predetermined heat-dependent expansive characteristics,thereby, under the clamping effect of the mechanical springs or clips30, increasing or maintaining the surface pressure on the semiconductorchip, so as to improve the thermal performance thereof.

Referring to the second embodiment of an electronic package 40 pursuantto the invention, as represented by FIG. 2 of the drawings, in whichelements which are similar to or identical with those in FIG. 1 areidentified by the same reference numerals, in that instance, rather thanproviding mechanical springs or clips, interposed between the surface 24of the cover plate 18 and each of the stiffener elements 32 is a furtheradhesive, preferably a thermal adhesive 42 which possesses a differentdegree of thermal expansion from that of the adhesive 26 which isinterposed between the semiconductor chip 12 and the facing surface ofthe cover plate or heat-sink 18. Thus, the further or second adhesive 42which is interposed between the stiffener elements and the facingsurface of the cover plate 18 or heat-sink has a lower coefficient ofthermal expansion than that of the adhesive 26 which is positionedbetween the upper surface of the semiconductor chip and the facingsurface of the cover plate. Thus, during operation of the electronicpackage, an increase in temperature will produce a greater degree ofexpansion of the thermal adhesive 26 between the semiconductor chip 12and the cover plate 18 relative to that of the second thermal adhesive42 between the stiffener elements 32 and the cover plate 18, therebyconstantly imparting a compressive force or pressure to thesemiconductor chip during operation of the electronic package 40.

It is noted that the effective thermal conductivity of the adhesivematerial, K_(eff), is a function of the intrinsic material ability toconduct heat, as well as the quality of the interfaces between thethermal adhesive material 26 and the semiconductor chip 12, and thecover plate 18. Any air gaps or voids which may be present in eitherinterface; in effect, between respectively the surface of thesemiconductor chip 12 and the thermal adhesive 26, and the thermaladhesive 26 and the facing surface of the cover plate 18 or heat-sink,will considerably decrease the effective thermal conductivity and,hence, increase the resistance to heat flow.

Recent experimentation and data has indicated that the thermalresistance is a function of force applied at the foregoing interfaces;in essence, if the adhesive 26 is maintained in a compressed stateduring the life of the product or electronic package 40, it will exhibita higher effective thermal conductivity. This data indicates that thethermal conductivity can be increased by a factor of up to 4, andpotentially even higher, illustrated hereinbelow in Table I.

                  TABLE I                                                         ______________________________________                                        Thermal Conductivity of                                                         Improved Vendor A, Vendor B and Vendor C Formulations                                 Sample         R.sub.-- m                                                                          k.sub.-- eff                                     Run # (Vendor) T(C) (C/W) (W/mk) Comments                                   ______________________________________                                        Vendor A bond thickness = 0.155 mm                                               1      A        33.5  0.175 1.40  moderate pressure                           2 A 36.7 0.179 1.34 moderate pressure                                             (repeatability)                                                           3 A 33.5 0.164 1.46 high pressure                                          Repeat after 1 week                                                              4      A        37.5  0.354 0.68  no pressure                                 5 A 38.0 0.254 0.94 moderate pressure                                         6 A 36.5 0.171 1.40 high pressure                                             7 A 30.5 0.189 1.27 release pressure                                              (4 hr after release)                                                      8 A 37.5 0.225 1.07 (12 hr after release)                                  Vendor B Bond Thickness = 0.16 mm                                                9      B        37.0  0.438 0.57  no pressure                                10 B 34.5 0.226 1.09 moderate pressure                                        11 B 34.0 0.208 1.19 high pressure                                            12 B 33.0 0.356 0.70 release pressure                                              (4 hr after release)                                                   Vendor C Bond Thickness = 0.1 mm                                                13      C        61.8  0.3   0.55  no pressure                                14 C 26.0 0.093 2.12 moderate pressure                                        15 C 35.5 0.077 2.72 high pressure                                            14 C 36.5 0.08B 2.27 moderate pressure                                        16 C 33.0 0.356 0.59 release pressure                                              (4 hr after release)                                                   Repeat after 1 week                                                             17      C        45.8  0.215 1.00  moderate pressure                          18 C 32.3 0.091 2.17 high pressure                                          ______________________________________                                    

All test samples were sandwiched epoxy positioned between two aluminumplates.

The measurements were implemented by clamping the sample between a hotand a cold plate, with a guard heater to prevent back heat loss. Thesite heat loss was contained by insulation to within 1.2%, as revealedby finite difference simulations of the tester. Approximately four hourswere required to reach steady state conditions through manualmanipulation of the guard heaters to contain the prescribed heat loss.The resistance was for adhesive plus two interfaces (to the aluminumplate) only.

The effective conductivity was calculated from the thermal resistancefor the given bond thickness.

The following is a summary of the data obtained (not necessarily in theorder of the tests performed). T(c)=mean sample temperature; R_(m)=thermal resistance o the sample (bulk adhesive plus two interfaces) forone inch area; k_(eff) =the effective thermal conductivity (bulk plustwo interfaces) assuming no change in initial bond thickness.

Calculations indicated that the uncertainty factor in resistance is lessthan 6% in all above measurements based on RMS analysis.

A NIST sample was tested for thermal resistance before and after theabove set and was found to agree with the NIST value within 3%.

In conclusion, it was ascertained that Vendor C sample has a capabilityof greater than 2 W/mK. When the surface voids are compressed, animprovement in the resistance is ascertained. In order to realize thedesired conductivity, the process needs to be improved in order toobtain good interface adhesive and/or positive pressure on theattachment. Release of pressure after high pressure reverted theresistance to a relatively poor value.

The Vendor A samples show the best value of about 1.4 W/mK. This valuewas the same as that obtained from a previous Vendor A formulationtested earlier, and the enhanced Vendor A formulation seemed to indicatesomewhat similar results.

In summation, the cover plate or heat-sink material required aselection, so as not to significantly creep through the period of thelife of the product. The clips attaching the cover plates to thesubstrate, as in the embodiment of FIG. 1, could be comprised ofmechanical springs, so as to always maintain a compressive load on theadhesive.

As mentioned hereinabove, in the embodiment of FIG. 2, the cover plateis attached to the substrate by a different adhesive from that usedbetween the chip and the cover plate, and whereby this adhesive has alower coefficient of thermal expansion than the previous adhesive, thechip to cover plate interface will remain in compression. Furthermore,since the chip to cover plate adhesive typically operates at somewhathigher temperatures than the adhesive between the stiffeners and thecover plate, this will aid in maintaining a compressive force or load,since the higher temperature will result in a greater expansion of thepreviously mentioned adhesive.

It is also possible to contemplate that a mechanical spring may bepositioned above the heat spreader or cover plate, but below the heatsink, thus, for example, in a multi-chip carrier, individual springloaded spreaders could be positioned on each chip and, in turn, thesewould be conducting heat to a heat-sink.

While the invention has been particularly shown and described withrespect to preferred embodiments thereof, it will be understood by thoseskilled in the art that the foregoing and other changes in form anddetails may be made therein without departing from the spirit and scopeof the invention.

Having thus described our invention, what we claim as new, and desire tosecure by Letters Patent is:
 1. An electronic package including meansfor applying a compressive force to the components of said electronicpackage during operation so as to enhance the thermal performancethereof, said electronic package components comprising a chip carrier, asemiconductor chip being mounted on said clip carrier, and a cover platepositioned over said semiconductor chip; said compressive force applyingmeans comprising means for clamping said cover plate to said chipcarrier, and a first adhesive being interposed between saidsemiconductor chip and said cover plate, said adhesive expandingresponsive to heat being generated during operation of said electronicpackage and maintains the compressive force on said semiconductor chip,stiffener members being positioned between said cover plate and chipcarrier adjacent opposite end surfaces of said semiconductor chip, saidclamping means comprising a second adhesive positioned between each saidstiffener member and the facing surface of said cover plate to maintainsaid semiconductor chip in clamped relationship between said cover plateand chip carrier.
 2. An electronic package as claimed in claim 1,wherein said first adhesive possesses a higher coefficient of thermalexpansion than the coefficient of thermal expansion of said secondadhesive whereby heat generated during operation of said electronicpackage will increase the compressive force acting on said semiconductorchip and enhance the thermal performance thereof.
 3. An electronicpackage as claimed in claim 1, wherein said first and second adhesiveseach comprise a thermal adhesive.
 4. A method of applying a compressiveforce to components of an electronic package during operation so as toenhance the thermal performance thereof, said electronic packagecomponents comprising a chip carrier, a semiconductor chip being mountedon said clip carrier, and a cover plate positioned over saidsemiconductor chip; said method comprising clamping said cover plate tosaid chip carrier, and interposing a first adhesive between saidsemiconductor chip and said cover plate, said adhesive expandingresponsive to heat being generated during operation of said electronicpackage and maintains the compressive force on said semiconductor chip,stiffener members being positioned between said cover plate and chipcarrier adjacent opposite end surfaces of said semiconductor chip, saidclamping being effected by positioning a second adhesive between eachsaid stiffener member and the facing surface of said cover plate tomaintain said semiconductor chip in clamped relationship between saidcover plate and chip carrier.
 5. A method as claimed in claim 4, whereinsaid first adhesive possesses a higher coefficient of thermal expansionthan the coefficient of thermal expansion of said second adhesivewhereby heat generated during operation of said electronic package willincrease the compressive force acting on said semiconductor chip andenhance the thermal performance thereof.
 6. A method as claimed in claim4, wherein said first and second adhesives each comprise a thermaladhesive.